The United States is considering a stepped-up military presence in Africa to pursue Islamic State group jihadists looking for new havens after the fall of their "caliphate," American officials say.

After IS lost its de facto capital Raqa in Syria this month, and its Iraqi stronghold of Mosul earlier, the group "has aspirations to establish a larger presence" in Africa, the US military's top officer General Joseph Dunford said on Monday.

From Libya to Egypt's Sinai, to East Africa and West Africa the jihadists have already posed a threat, Dunford, Chairman of the Joint Chiefs of Staff, told a press conference.

He was discussing the October 4 clash in Niger, West Africa, that cost the lives of four American soldiers.

Along with five Nigerien troops, the US soldiers died on the Niger-Mali border in an attack carried out by locals associated with IS, according to Dunford.

The incident shocked many Americans unaware of the hundreds-strong US military presence in that country.

Dunford said the military will make recommendations to President Donald Trump and Defense Secretary Jim Mattis "for the allocation of forces that meet what we see as the threat, what we anticipate the threat to be."

On Tuesday he meets military chiefs from 75 countries "to talk about the next phase of the campaign" against IS.

Speaking to reporters following a meeting with Mattis last Friday, Republican Senator Lindsey Graham, a member of the Senate's Armed Services Committee, said bluntly: "The war is morphing. We're going to see more actions in Africa, not less."

After the Middle East, Africa already has the greatest presence of American special forces. Official figures show more than 1,300 of the troops are deployed there.

These elite units train local forces in counter-terrorism and "will only accompany those forces when the prospects of enemy contact is unlikely," Dunford said.

These rules of engagement "are going to change when it comes to counterterrorism operations," Graham said.

He hinted that American troops would be authorized to shoot first on "terrorist" targets, which is not the case now.

A growing threat

Chairman of the Joint Chiefs of Staff General Joseph Dunford says the Islamic State group aspires to a greater presence in Africa

(AFP/File)

The European Union's presidency also warned this month that countries in that bloc must monitor "very carefully" a growing IS threat in North Africa, where fighters have relocated.

Dunford said the war is moving to multiple arenas.

"I'm not sure I'm ready to say it's shifting just to Africa. We're dealing with a challenge that exists from West Africa to Southeast Asia," he said.

"I believe ISIS will attempt to establish a physical presence outside of Iraq and Syria" after losing Mosul and Raqa, the general added, using another acronym for IS.

"That's exactly why we're conducting the kinds of operations we're conducting in Niger, to ensure that local forces have the capability to prevent that from happening."

The US supports with aerial refueling and intelligence France's Operation Barkhane against jihadists in five Sahel nations: Mauritania, Mali, Chad, Niger and Burkina Faso.

In all, the US military has about 6,000 personnel in 53 African countries, Dunford said.

According to a report to Congress by General Thomas Waldhauser, who heads the US Africa Command (AFRICOM), the American presence is notable in Chad, the Democratic Republic of Congo, Ethiopia, Somalia, Uganda, Rwanda and Kenya.

The numbers of US special forces varies but the Niger contingent of about 800 is currently the largest in any one country on the continent.

Dunford vowed the US will remain, despite the four fatalities in Niger.